1. Field of the Invention
The present invention relates to dinitroalkylphenols, methods for making the dinitroalkylphenols and methods for using the dinitroalkylphenols as polymerization inhibitors.
More particularly, the present invention relates to compositions comprising 2,6-dinitro-4-alkylphenols and mixtures of 2,6-dinitro-4-alkylphenols and 2,4-dinitro-6-alkylphenols where the mixture comprises the natural isomer mix derived during phenol alkylation, polymerization inhibitors composed of 2,6-dinitro-4-alkylphenols and mixtures of 2,6-dinitro-4-alkylphenols and 2,4-dinitro-6-alkylphenols, methods for making and using same.
2. Description of the Related Art
Alkylated dinitro phenols or hydroxy aromatics are used to inhibit olefin polymerization by deactivating radicals that lead to polymerization. Generally, when phenolic compounds are alkylated, the resulting product mixture does not contain a single product. For example, in the manufacture of 2-alkylphenol, the manufacturing process requires some type of separation step to remove undesired by-products, namely 4-alkylphenol, prior to selling a substantially pure 2-alkylphenol as an inhibitor.
Thus, there is a need in the art for more cost effective polymerization inhibitors and more cost effective methods for preparing inhibitors, where alkylation and nitration can be performed without concern for product separation or purification, including the separation of monoalkylated phenols.
The present invention addresses these needs by providing dinitroalkylphenolic polymerization inhibitors of formula (I): 
where R is an alkyl group having between about 3 and 12 carbon atoms, preferably 3 to 6 carbon atoms.
The present invention also provides a vinyl polymerization inhibitor comprising a mixture of dinitroalkylphenolic polymerization inhibitors of formulas (I) and (II): 
where R is an alkyl group having between about 3 and 12 carbon atoms, preferably 3 to 6 carbon atoms.
The present invention provides a method for making a mixture of inhibitors of formulas (I) and (II) including the steps of alkylating phenol with an olefin having between about 3 and about 12 carbon atoms under conditions sufficient to promote the alkylation and form an alkylated phenol composition comprising mono-alkylated phenols and minor amount of di and tri alkylated phenols. The unwanted formation of di and tri alkylated phenols can be minimized by controlling the extent of phenol conversion, i.e., allowing only partial phenol conversion coupled with phenol recycle. The alkylated phenol composition is then nitrated under conditions sufficient to form a nitrated phenol composition comprising dinitrated mono-alkylated phenols and minor amount of mononitrated dialkylphenols and other nitrated species. The resulting nitrated composition comprises predominantly 2,4-dinitro-6-alkylphenol and 2,6-dinitro-4-alkylphenol. Alternatively, the method can also include the step of separating the monoalkylated phenols from any di or tri alkylated phenols prior to nitration, unless the di and tri alkylated phenols comprises less than about 5 wt. % of the entire composition.
The present invention also provides a vinyl monomer stabilized composition comprising a vinyl monomer and an inhibitor system of the present invention and to a method for inhibiting vinyl monomer polymerization including the step of adding an effective inhibiting amount of an inhibitor comprising a mixture of dinitroalkylphenolic polymerization inhibitors of formulas (I) and (II).